President Hamid Karzai has rejected an American apology for the killing of nine Afghan boys in a Nato air attack and said civilian casualties were no longer acceptable. According to a statement from his office, Karzai told General David Petraeus, the commander of coalition forces in Afghanistan, that expressing regret was not sufficient for the killing of the boys, aged 12 and under, by coalition helicopters. Nato has also apologised for the deaths.

Civilian casualties from coalition operations are a major source of strain in the already difficult relationship between Karzai's government and the US, and they generate widespread outrage among the population. "Regrets and condemnations of the incident cannot heal the wounds of the people," the statement said.The boys were killed on 1 March when helicopters were deployed after an attack on a base in the Pech valley of Kunar province.

General David Rodriguez, who directs day-to-day operations of coalition forces across Afghanistan, later issued a video apology. Rodriguez said troops at a base in the valley were responding to a rocket attack and dispatched helicopters to the location they were told the rockets came from. He said the helicopters thought they were engaging insurgents, but it later turned out they were boys from a nearby village who were cutting firewood.

At a meeting of the National Security Council, a body that includes cabinet members and senior political and military officials, Karzai asked Petraeus to prevent such incidents.

Nato operations against insurgents in eastern Afghanistan have caused friction with the Karzai administration in recent weeks after government charges that the military has caused a number of civilian deaths and casualties.

Earlier this month, the government claimed 65 civilians, including 40 children, were killed in a Nato assault in Kunar. NATO, however, said that video of Kunar operations showed troops targeting and killing insurgents, not civilians.